In mobile data communications, when a mobile node moves from one mobile communication region to another mobile communication region, handoff technology is required to allow the continuous mobile data communication. Examples of handoff for mobile IP are intra-subnet handoff, inter-subnet handoff, and inter-router handoff.
According to these handoffs, since the intra-subnet handoff where a mobile node moves into an adjacent cell corresponding to an administration address region of the same router uses the same IP subnet address, a new IP subnet address is not required.
On the contrary, in a case of the inter-subnet handoff and inter-router handoff where the administration address region of the router changes, a new IP subnet address is required in a moving mobile node. That is, when a mobile node visits another region with a different IP address in a mobile IP network, a foreign agent (also, referred to as a FA) operating as a virtual router in the another region provides a care-of-address (CoA) to the mobile node, and the mobile node registers the CoA in a home agent through the foreign agent. The home agent encapsulates data transmitted toward the mobile node and transmits the encapsulated data as the CoA, and then the foreign agent unpacks the data and outputs the unpacked data into the mobile node. At this point, the foreign agent operates as a default router of the mobile node for the data that is transmitted by the mobile node. When the handoff requiring an address change occurs, a new CoA is required for a mobile node. Accordingly, an additional method for processing a mobile IP is required.
On the other hand, if a mobile node moves into an adjacent cell, handoff of a link level occurs between stations (Base Transceiver Station (BTS) of Global System for Mobile communications (GSM) or BS of Code division multiple access (CDMA)). This high-speed handoff of a link level has various preventive measures such as soft handoff, etc. The conventional handoff broadcasts the handoff over the cell with the mobile node and adjacent cells. As a result, communication resources are wasted, and a packet delivery is delayed because an operation for correcting the mobile node's position information is required in advance. Thus, there is some difficulty in directly applying this method to the mobile node providing an IP service.
There have been various proposals in resolving the above limitation. For example, referring to U.S. Patent publication No. 2001/0036834 A1 (2001. 11. 1), when a mobile node notifies a subnet agent that a handoff operation may occur, the subnet agent delivers this to a mobile agent (including a home agent and a foreign agent), and the mobile agent determines subnet agents adjacent to a subnet agent with the mobile node and sets the adjacent subnet agents as a multicast group. That is, according to the above proposed method, high-speed handoff can be obtained by multicasting packets that need to be delivered to the mobile node.
However, according to the conventional proposal, resource waste may still occur because the data packet is delivered to an unnecessary subnet agent during a predetermined time. Additionally, since a control signal path for the handoff is identical to a data path used for delivering the data packet to the mobile node, the control signal packet is followed by the data packet in a buffer. As a result, hand-of-line blocking phenomenon occurs because the control signal packet is processed later on.
According to another method, as being discussed in Internet Engineering Task Force (IETF), a gateway foreign agent (GFA) maybe introduced as upper level of the level of the foreign agents with respectively different address regions, such that the mobile node utilizes the CoA provided from the GFA. However, this method may not be directly applied to conventional mobile nodes that do not identify the GFA.
Additionally, although the GFA is used, in a case of when a control signal packet for processing handoff and data packet between mobile nodes are utilizing the same path, as described before, head-of-line blocking phenomenon, i.e., property of the IP network, still exists. Additionally, the mobile node needs to be registered as a conventional normal home agent and as the GFA even if the GFA exits.
To compensate the above limitations, when the mobile node requests registration to a home agent through a foreign agent, an IP address of the GFA is added to an extension field of a registration request message in the mobile node, which is transmitted from the foreign agent, by placing the GFA between the foreign agent and the home agent. Additionally, when the mobile node moves from the foreign agent in area A from the foreign agent in area B, the GFA returns back a response message if the mobile node is registered mobile node when identifying the registration request. In a case when the mobile node is not registered, the GFA generates and adds a new visitor entry to a visitor list inside the GFA and also delivers registration request message to the home agent of the mobile node by adding an IP address of the GFA to the extension filed of the registration request message in the mobile node moving from the foreign agent of the region B. In this case, although there is advantage in that the mobile node may use a conventional procedure that registers the mobile node as a normal home agent, all the home agent, the GFA, the foreign agent need to recognize that there is an address of the GFA in an extension field of a registration request message and also need to be capable of processing the address. Additionally, in a case of when the mobile node is not registered, when a deny message of binding update is received, the processes of generating and adding a new visitor entry to a visitor list inside the GFA before receiving a response message for the registration request from the home agent need to be erased again. Furthermore, in a case of when the reply of the registration request is late from the home agent, if the mobile node moves to another foreign agent region for the mean time and then again requests registration, the GFA already exists in the visitor list. As a result, it may be venerable to malicious attack because the GFA may deliver information when the mobile node is not actually registered. Furthermore, because all data packets always pass through the GFA, the load of the data delivery path may increase.